Biosecurity is the implementation of a set of management, behavioral, and physical measures designed to decrease the risk of introduction, establishment, and spread of pathogenic agents to, within, and from a population. Biosecurity includes all activities involving domestic, captive/exotic, and wild animals and their products.
Bioexclusion centers on the prevention of disease introduction and relies on external biosecurity practices. Biocontainment centers on preventing the spread of disease within a farm or group of animals, or to other farms or groups of animals, and relies on the implementation of internal biosecurity practices. Thus, disease control and prevention rely on the interrelated processes of bioexclusion, surveillance, and biocontainment.
Prevention of disease is costly, difficult, and time-consuming, and it is directed primarily at preventing epidemic or exotic diseases. It involves preventing the disease-causing agent from being introduced into an individual or a population and might involve the eradication of disease-causing agents from a population of animals or from a geographical area.
In contrast, control programs are less demanding and focus primarily on limiting endemic diseases to tolerable levels within a population of animals or a geographical area. Although preventing exposure to disease-causing agents remains an objective of control strategies, strategies focus primarily on limiting the consequences of disease.
Disease Transmission in Biosecurity
Understanding disease transmission is central to designing proper biosecurity protocols. Diseases can be transmitted in many ways, and direct animal-to-animal contact, as well as contact with contaminated fomites, is one of the most common transmission routes. Contaminated semen and natural mating can be sources of sexually transmitted diseases.
Many fomites (inanimate objects) are carriers of disease-causing agents. The survival of disease-causing agents on fomites might depend on the ability of the agent to survive outside the host, on the composition of the particular fomite, and on how easily the fomite can be disinfected. Examples of fomites that are considered high-risk include trailers, vehicles, maintenance and repair tools, boxes, materials used to remove dead animals, and loading chutes.
Vectors are also capable of transmitting diseases. Among the most important vectors are birds, bats, rodents, feral and wild animals, stray and domestic animals, and insects.
Air can be a source of diseases, particularly in areas of high animal density. Contaminated water and feed and consumption of contaminated, raw, untreated animal products have also been implicated in disease transmission.
Manure, bedding, and carcasses can also be a source of disease-causing agents, unless disposed of properly.
Finally, humans can act as both mechanical and biological vectors, and training and awareness of personnel working with animals is necessary for proper implementation of biosecurity programs.
Disease Prevention in Biosecurity
Disease prevention depends on stringent bioexclusion to avoid contact between the disease-causing agent and the host, early detection of breaches in biosecurity through vigilant surveillance, and rapid implementation of a ruthless biocontainment policy. Meeting these requirements is feasible only if there is an effective way to detect infection; containing the infection through slaughter or other means, clean-out, and disinfection; and preventing dissemination of the disease-causing agent.
Eradication is reserved for diseases that pose a dire public health threat, that have a devastating effect on animal performance and global trade, or that severely compromise end product quality. Elimination of diseases without a regulatory framework is common in food animals if these diseases are economically important and their elimination is advantageous to producers.
Many biosecurity measures aimed at preventing or eradicating epidemic disease also produce beneficial by-products, such as establishment of a firm foundation for control of endemic diseases and enhancement of host resistance through immunization.
Disease Control in Biosecurity
In disease control strategies, the emphasis shifts from preventing disease to decreasing its consequence or economic impact. Prevalence data are now used primarily to assess the level of protection and challenge, not merely the presence or absence of disease.
Although biosecurity still relies on principles of prevention, disease control programs focus more on limiting the extent of disease and consequences of exposure.
Disease Management and Disease Determinants in Biosecurity
Disease risk management must be an integral part of any animal management program. Economic analysis is a critical step in biosecurity plan design because resource allocation must be aligned with risk. The success of a disease control program depends on the ability to identify and subsequently address the risk of infection.
Disease risk in a population is characterized by the probability of new cases of disease occurring in a disease-free population over a specific period. Aggregate risk is the sum of each individual risk of adverse health effects in an exposed population.
The spread and consequences of new infections are influenced by several factors, referred to as disease determinants. Because infectious disease is the result of complex interactions between several factors, any factor that influences the risk and consequence of disease challenge is a disease determinant.
Disease determinants have historically been classified as follows:
Primary or secondary: Primary determinants (eg, viruses or bacteria) directly cause disease. Secondary determinants, such as genetics, influence susceptibility or resistance to disease.
Intrinsic or extrinsic: Intrinsic determinants are characteristic of the host or the disease-causing agent. Extrinsic determinants are external factors such as the environment.
Host associated, agent associated, or environment associated. In intensive production units, the housing environment, agent, and host determinants are largely under the control of the manager or caretaker, who thus has the greatest influence on disease determinants.
Risk Assessment in Biosecurity
Risk assessment is used to estimate the probability of exposure to a disease-causing agent, the probability that exposure will cause infection and disease, the probability that disease will spread, and the consequence of such spread.
Statistical techniques such as the chi-square test can be used to assess whether a specific factor or process is correlated with disease; however, these tests provide no estimate of the degree of disease risk.
The measure of association most frequently used to assess risk magnitude is the relative risk, or risk ratio:
RR = P(Disease|Exposed) ÷ P(Disease|Unexposed)
where RR is the risk ratio and is calculated as the probability of disease given exposure divided by the probability given no exposure (1).
Disease control begins with evaluating each part of the production process as a risk factor for infection. On a basic level, risk of infection equals the probability that each event will cause infection multiplied by the number of times each event occurs. However, estimating the degree of risk requires analysis of many other factors, including host resistance and the dose or virulence of the organism.
As a first step to limiting health risk, a biosecurity program should critically assess the necessity of all events or processes that potentially carry risk, and only crucial events and processes should be allowed. Limiting the potential for infection within crucial events focuses on improving host resistance or decreasing the challenge dose or virulence of the infecting organism.
Host, Environmental, and Pathogen Factors in Disease
Biosecurity is all procedures implemented to decrease the risk and consequence of infection with a disease-causing agent. This broad definition recognizes that disease is a complex interaction between the host, the disease-causing agent, and the environment.
Host Factors in Disease
Immune efficiency is the key factor that determines host resistance. A healthy animal produces an appropriate immune response sufficient to combat infection and its impact on productivity. Conversely, a response that is either excessive or insufficient will adversely affect well-being and performance.
Resistance to infection varies among individuals primarily because of genetic differences.
Immunosuppression decreases both individual immunity and herd or flock immunity. The weakest members of a population are the most negatively impacted by individual stressors (eg, disease or poor nutrition), which have a cumulative impact on immunity. These highly susceptible animals tend to skew the resistance curve to the right, resulting in a dramatic decline in flock or herd immunity.
Environmental Factors in Disease
Environmental factors modify the exposure to a pathogen, thereby determining whether a sufficient dose is received to overcome the host's immune defenses.
The standard measure of infectivity is the infective dose 50 (ID50), the minimum dose (eg, number of infective organisms or viral particles) required to generate infection in 50% of the animals in a specific population exposed to the disease-causing agent.
When designing a health program, it is important to remember that the ID50 represents more than a measure of disease virulence. Because the risk of a challenge exposure increases after one animal in a population has become infected or diseased, ID50 represents the dose required to infect the least resistant animal within the population.
Pathogen Factors in Disease
Virulence is the disease-causing agent's inherent infectivity (capability to infect) and pathogenicity (capability to cause disease) within the host.
Agent replication increases the risk of exposure by increasing the dose and (possibly) virulence of the disease-causing agent, with each new infection escalating the risk further, until even relatively resistant animals become at risk.
Epidemiology in Biosecurity
Effective biosecurity requires an understanding of the causative relations between exposure and disease, because the prevalence and consequence of any infectious disease involves complex interactions between several disease determinants.
The epidemiology and relative risk of infection for each disease should be assessed to determine the best way to allocate resources for effective control procedures. Furthermore, epidemiological statistics can be used to determine the best way to limit current and future financial risk. For a more in-depth review, see Principles of Epidemiology.
Several important epidemiological factors must be considered in biosecurity. In addition to identifying animals that have become infected, other variables associated with the relative importance of a particular source should be assessed, including shedding pattern, host range, mode of transmission, and farming practices.
Investigating the source of infection is important to control disease outbreaks and protect populations from new infections.
Disease transmission (the act of an infectious agent moving from a reservoir to a new host) within a population is influenced by direct contact with infected animals, as well as by indirect exposure from contaminated objects (fomites) and aerosols. These forms of horizontal transmission influence the rate and extent of transmission within and among groups.
In vertical transmission, diseases are transmitted between parent and offspring. Vertical transmission can result from direct contamination or (in some cases) from transovarial transmission of disease-causing agents within an embryo or from disease-causing agents crossing the placenta and affecting fetuses.
Disease spread (the overall increase in the number of cases of a disease in the population over time) is influenced by factors such as incubation period, replication rate, resilience, virulence of the disease-causing agent, and contact rates between infected and susceptible individuals. These factors influence spread within the population, as well as disease course (acute, subacute, chronic) within individuals. For example, disease spread is accelerated by a resilient organism with a short incubation period and high replication or shedding rate.
Disease spread between herds or flocks should also be considered, particularly in areas of high farm or animal density. Factors such as climate, ability of the disease-causing agent to survive in the environment, and topography are known to influence disease spread between populations.
Susceptibility and predisposition to disease are important epidemiological factors. For an effective control program, it is important to know the range (species, breed, type) of susceptible hosts. Furthermore, host, agent, and environmental factors can predispose animals to infection or disease. For example, environmental stress can compromise immune function.
Disease prevalence is directly proportional to the risk of challenge. Endemic diseases are difficult to prevent. Exotic (ie, not normally present) or epidemic diseases are easier to track, and potentially to contain and eradicate, if proper biocontainment mechanisms are in place.
Morbidity refers to animals in a population that show clinical signs of disease at particular times. Diseases that spread rapidly typically have a high morbidity rate (percentage). Mortality rate represents the percentage of animals in a population expected to die during a particular disease outbreak. Individuals might recover from disease, and recovery can be influenced by many factors.
Key Points
Many factors intrinsic to the host, the disease-causing agent, and the environment can contribute to disease.
Basic knowledge of epidemiology, transmission, host susceptibility, and disease management is crucial to designing effective biosecurity programs.
For More Information
Thrusfield M, Christley R, eds. Veterinary Epidemiology. 4th ed. Wiley Blackwell; 2018.
Gopal Narayan D, Sinha DK, Singh DK, eds. Veterinary Public Health & Epidemiology: Epidemiology-Veterinary Public Health-One Health. Springer; 2023.
References
Thrusfield M, Christley R, eds. Veterinary Epidemiology. 4th ed. Wiley Blackwell; 2018.
